The typical vertical conveyor includes a carriage having a platform to support the cargo and the carriage is guided for vertical movement between the lower and upper levels on a supporting structure or frame that includes a pair of spaced vertical columns. With a hydraulically actuated conveyor, one or more hydraulic cylinder units are each connected through a cable or chain mechanism to the carriage and through extension of the cylinder units, the carriage can be raised to the upper level. To lower the carriage, operation of the pump motor of the hydraulic system is terminated and valving is actuated to permit discharge of the hydraulic fluid from the cylinder units and consequent controlled lowering of the carriage.
A typical mechanically operated vertical conveyor utilizes an electric motor that is connected through a chain drive to the carriage. Operation of the reversible motor will act to raise and lower the carriage. As a safety feature, the typical vertical conveyor includes an overload protection mechanism which will sense an overload on the carriage as the carriage is elevated and terminate operation of the drive system.
The conventional vertical conveyor has been dependent solely on the lift mechanism to support the carriage at the upper level. Because of the natural elasticity of either the cable or chain, which connects the drive unit to the carriage, and due to the flexing of the hydraulic hoses in a hydraulically operated conveyor, the conveyor has been found to move as much as an inch and one-half as it is loaded or unloaded at the upper level. To deal with this problem, many hydraulically operated vertical conveyors have incorporated a pressure switch to stop operation of the hydraulic pump motor when the carriage is at its upper level. The pressure switch builds pressure within the hydraulic system, thereby significantly reducing the stretch of the lifting cables or chains and the flexing of the hydraulic hose. However, the pressure which is built up through use of the pressure switch bleeds off in a relatively short period of time, either because of oil leaking past the hydraulic cylinder seals or the oil leaking through the check valve, so that after the pressure is bled off, the floating condition will reappear. As a result, unintentional descent of the carriage from an upper level can occur, creating a severe safety hazard.
In an effort to address this problem, U.S. Pat. No. 5,228,537, issued Jul. 20, 1993 to Pfleger et al and assigned to the assignee of this application, discloses a safety mechanism for automatically locking a vertical conveyor at an elevated or upper level to thereby prevent uncontrolled descent of the carriage. The conveyor comprises a carriage having a platform adapted to support a load of cargo, and the carriage is guided for vertical movement on a supporting structure frame and includes a pair of spaced vertical columns. The carriage is moved between the upper and lower levels by a drive mechanism which preferably consists of one or more hydraulic cylinders which are mounted on the supporting frame and which are connected through a cable and sheave arrangement to the carriage.
The safety mechanism of the aforementioned patent includes a pair of locking bars that are mounted on the opposite sides of the carriage and are adapted to engage ledges or abutments on the vertical columns of the supporting frame when the carriage is at an upper, elevated level. As the carriage moves upwardly, it passes slightly past the upper level to a higher position and actuates a limit switch on the supporting frame. Actuation of the limit switch terminates operation of the pump motor of the hydraulic system and actuates the valving to permit the hydraulic cylinders to lower the carriage. At the higher position, the locking bars are moved from a retracted or release position to a locking position so the locking bars can engage abutments on the vertical columns as the carriage moves downwardly from the third higher position to the upper level.
To move the carriage from the upper level or second level back down to the lower level, the cylinder units are actuated, causing the carriage to move upwardly from the second level to the third higher position and again actuate the limit switch to terminate operation of the pump motor, thus enabling the carriage to lower. Simultaneously, the locking bars are moved from the locking position to the release position, so the carriage can then descend through the upper level back to the lower level.
More particularly, the locking bars are pivotally connected to opposite sides of the carriage and are biased outwardly to the locking position where they can engage the abutments on the respective columns of the supporting frame. When the carriage is at the lower level, a follower on each locking bar is engaged with a projection or holding member on a slide that is movable relative to the carriage, and the engagement of the follower with the projection holds the locking bar in the release or retracted position.
When the carriage is elevated slightly above the upper level to the third position, the slide and the carriage engages a stop which moves the slide downwardly relative to the locking bar, thereby moving the projection out of engagement with the follower and permitting the locking bar to pivot outwardly to the locking position under the influence of the biasing mechanism. As the carriage then lowers from the third position to the upper level, the locking bars will engage the abutments on the column to hold the carriage at the upper level.
While the above-described arrangement has generally been satisfactory, problems sometimes arise in the user's installation of the safety mechanism which causes the locking bars to malfunction. In particular, depending on the installed location of the stop mounted on each vertical column of the supporting frame, the locking bars are susceptible to remain in a retracted position relative to the abutments so that the locking function may be inconsistent which resurrects a safety hazard.
In an effort to rectify this complication, it would be extremely desirable to provide an improved safety mechanism which eliminates the use of the installed stop in order to improve the operation of the vertical conveyor. Likewise, it would be desirable to simplify the multi-component locking arrangement used in the prior art. At the same time, the safety mechanism should continue to provide an overload protection mechanism for sensing an overload on the carriage as the carriage is elevated to discontinue operation of the drive mechanism. Furthermore, the improved safety mechanism should stabilize the carriage against lateral shifting relative to the supporting frame and prevent downward float to the carriage as a load or cargo is applied to the carriage.